The present invention relates generally to pyroelectric infrared (PIR) sensors, and more specifically to PIR sensors used to detect direction and speed.
PIR sensors are well known in the art. These sensors are commonly used in security systems for measuring motion in a monitored area. PIR sensors use materials having a pyroelectric effect. One common material is LiTaO.sub.3, which in its crystalline form is spontaneously polarized (i.e. electrical dipoles in the crystal structure develop). Heating the LiTaO.sub.3 crystal to a temperature just below its Curie temperature in an electrical field causes these dipoles to line up in a direction of the electrical field.
Bringing opposing electrodes into contact with the polarized crystal causes the surface to be electrically charged. Ions in the air neutralize this surface charge. Thereafter, the crystal absorbs incident infrared radiation (IR). The absorption of the IR causes the temperature of the crystal to change, altering the spontaneous polarization and thus the number of dipoles. The change in the number of dipoles unbalances surface charges on the crystal's surface. It is possible to measure this surface charge imbalance as a voltage change. Thus, voltage changes on the surface of the crystal are indications of incident IR. The voltage change and the subsequent detection of IR required a temperature change in the crystal. To include a temperature change in the crystal requires either a moving IR source, or a chopper disposed between the crystal and the IR source.
FIG. 7 is a schematic presentation of the pyroelectric effect. At the top of FIG. 7 a source of IR is shown modulated by a chopper. The chopper first is closed, then opened to illuminate a pyroelectric crystal with the infrared radiation, and then subsequently closed. Below the status graph of the chopper is seen the charge distribution of the crystal surface corresponding to the chopper status. Position A, with the chopper closed prior to illumination, shows a balanced surface charge aligned with an electric field (not shown). Position B, opening the chopper, disturbs the balance of the dipoles, producing a net positive charge distribution. Position C occurs sometime later with the chopper open, after excess surface charge is neutralized and charge becomes rebalanced at the new dipole generation rate. Thereafter, at position D (closing the chopper), induces an equal but opposite charge distribution in the crystal. Ions become associated with the unbalanced crystal charge at position E, sometime later. The graph below the representations of the crystal charge distribution illustrates output from a sensor employing such a crystal.
FIG. 8 is a schematic diagram of a prior art PIR detector 100 consisting of dual pyroelectric elements (LiTaO.sub.3) 102a,b a high-ohmic resistor 104, and a low-noise field effect transistor (FET) 106 built into a TO-5 package. The TO-5 package includes a window 110 made up of a silicon filter which limits incident rediation to wavelengths in a prespecified range.
The prior art employed dual element sensors to reduce noise signals from the sensor. FIG. 9 is a top view of a PIR sensor 100 having two sensing elements 102a and 102b. Typically, the sensing elements are one millimeter by two millimeters and separated by a one millimeter space. The polarities of the sensing elements are reversed as shown., with the sensing elements oriented at about a forty-five degree angle to improve the noise reduction feature of the sensing element.
It is common in the prior art to employ these PIR sensors in switches to control room lighting, for example. In one common application, a switch with a PIR sensor is mounted to monitor an entrance to a room. When a person enters the room, the PIR sensor detects the entrance and begins operation of a timer. Subsequent detections of persons entering or leaving the room will reset the timer. When the timer expires, the switch automatically turns the lights off. This is a desirable energy-saving feature. These switches do not have an ability to detect relative motion of the person entering or leaving the room, so that a person leaving the room will reset the timer. If the switch were able to discriminate between a person entering or leaving, the switch could immediately turn the lights off rather than waiting for the timer to expire.